CED coating is a well-known process of coating electrically conductive substrates, in particular, metallic substrates. In CED coating, the workpiece is connected to the direct current source as a cathode and the CED coating film is then deposited on the substrate surface by the current. The CED coating film is then thermally cross-linked.
In CED coating, the substrates are generally initially subjected to a pre-treatment and then passed to the CED coating process. The substrates are coated in a conventional CED coating bath (CED coating composition, CED coating agent, CED coating paint), and excess, inadequately adhering CED coating composition is then rinsed from the substrates back into the CED coating tank. To avoid an increase in the volume of the CED coating bath, the substrates are conventionally rinsed with ultrafiltrate obtained from the CED coating bath.
The ultrafiltrate is obtained by known methods and, in addition to water, contains, for example, low molecular constituents, neutralizing agents, solvents and dissolved salts from the CED coating bath.
Finally, the substrates can be rinsed with water.
A process is described in U.S. Pat. No. 4,655,787 in which, by using the distribution principle, additives are introduced into an uncured CED coating film. In this process, substrates having a CED coating film are immersed in a dipping solution which contains the appropriate additive. The additive is absorbed within the CED coating film in the process. UV stabilizers, antioxidants, plasticizers, corrosion inhibitors, anti-static agents and in particular, dyes are used as additives. These are each low molecular materials with low water solubility and are dissolved in a solvent medium which contains water, water-miscible solvents and a hydrotrope. Salts and urea compounds are mentioned as hydrotropes and facilitate transition of a dye from the dipping solution into the non-cross-linked CED coating film by a salting-out effect. Once the distribution process has progressed to the desired extent, the substrate is rinsed and subjected to a baking process.
The post-treatment of non-cross-linked CED coating films prior to baking with an oil-in-water emulsion of water-insoluble, polymeric, non-ionic surface defect-counteracting agents to prevent the formation of surface defects, such as craters, in the CED coating film as taught in DE 43 03 812.
DE 43 03 787 describes the post-treatment of uncured CED coating films prior to baking with an aqueous polyvinyl alcohol solution, likewise with the goal of preventing the formation of surface defects in the CED coating film.
Metal compounds, for example, specific metal salts, are used in CED coating compositions, in particular, as cross-linking catalysts and/or anti-corrosion additives. In particular, lead compounds, such as lead silicate, often also in combination with tin compounds, such as dibutyl tin oxide, have been used. More recently, lead-free CED coating compositions have become known. CED coating compositions containing bismuth compounds have acquired particular significance, as described, for example, in U.S. Pat. No. 5,936,013, U.S. Pat. No. 5,702,581, U.S. Pat. No. 5,554,700, U.S. Pat. No. 5,908,912, U.S. Pat. No. 6,174,422, U.S. Pat. No. 5,670,441, WO 96/10057, U.S. Pat. No. 5,972,189, WO 00/50522, U.S. Pat. No. 6,265,079, EP 1 041 125, WO 00/47642, WO 01/51570, DE-A 44 34 593 and U.S. Pat. No. 5,702,581.
It has been found that the metals desired in the CED coating film and incorporating into the CED coating film as a constituent of the CED coating composition do not have to be present as metal compounds in the CED coating composition. They may be passed onto and or be passed into the still non-cross-linked CED coating film after CED coating, before subsequent thermal cross-linking. This can be accomplished by bringing the still non-cross-linked CED coating film into contact with an aqueous preparation of one or more appropriate metal compounds.
An advantage of this invention is that the production of CED coating compositions can be simplified and there is a greater freedom for the formulation of CED coating compositions.
Finally, components which are incompatible with the at least one metal compound can thus also be incorporated into the CED coating composition as a constituent of its formulation.
In general, problems, in particular stability problems, as can occur in dispersions containing metal ions, can be avoided in the CED coating composition.
The process according to the invention also allows at least one further different metal to be subsequently added onto and/or into a still non-cross-linked CED coating film containing one or more metals, after CED coating.